Add some underscore methods.
Add course UI.

New UI iteration that combines lesson view into course view.
/lessons/:id loads the correct (common) UI.
Refactor heading component.
Various small style improvements.
List courses on homepage or redirect if only one.
Add stylized lessons dropdown.
Fix regression: sub-tabs need to use this.props.updateTabContent().
Simplify React state by eliminiting main tabs.

Previously the primary tabs kept track of every tab by namespacing the data. It's easier to define a 'primary content' area and just have the tabs continually create and destroy this content.

It gives js a chance to clean up memory as well (hopefully).
this.updatePrimaryContent should take a mergeable hash.
Default to the latest (upcoming) lesson.
Standardize Error rescue control flow.
Split logged in home screen vs logged-out.
Add flash notice to login screen.
Add not_found page.